Directional coupler



March 24, 1953 H. J. RIBLET 2,632,809

l DIRECTION/4p COUPLER l Filed Nov. 5, l'1947 8 sheets-sheet 1 March 24, 1953 H'. .1. RIBLET 2,632,809

DIRECTIONAL couPLER Filed Nov. 5, 1947 8 sheets-sheet s FVG. 6

/V '-"RELAT/VE NUMBER OF D/P5 /NvE/wvre Hf/v'fy J. maur A TURNE Y- H. J. RIBLET DI'RECTIONAL COUPLER March 24, 1953 Filed Nov. 5, 1947 8 Sheets-Sheet 5 Hawk y J. m5257- H. J. RIBLET DIRECTIONAL COUPLER March 424, 1953 Filed Nov. 5. 1947 /Nve/VTOR Hf/wry J. /r/HLET Bv /N CENT/METERS H. J. RIBLET March 24, 1953 DIRETIONAL couPrlER 8 Sheets-Sheet '7 Filed Nov. 5, 1947 March 24, 1953 H. J. RIBLET 2,632,809

DIRECTIONAL COUPLER ratio, in the region of unity.

- the present invention;`

Patented Mar. 24, 195.3

ES PATENT OFFICE DIRECTIONALA COUPLER Henry J. Riblet, Belmont, Mass., assignor to Raytheon Manufacturin of Delaware g Company, a corporation Application November 5, 1947, Serial No. 784,277

13 Claims.

The present invention relates in general to waveguide directional couplers, and in particular to a new type of directional coupler having 4improved performance and structural characteristics.

It is an object of the present invention to provide a waveguide directional coupler having a coupling characteristic which is substantially i constant over a, relatively wide band of operating frequencies.

It is another object to provide such a directional coupler wherein a coupling characteristic havingV a desired value lying between wide limits can be provided, and which can consequent- .ly be employed as a balanced bridge having the characteristics of a magic T or rat race bridge, without being frequency critical.

It is another object to provide such a directional coupler having high directivity, of the order of 40 decibels or more.

It is another object to provide such a directional coupler having a low input standing wave It is still another object to provide such a directional coupler wherein the structural requirements to provide. broad band characteristics are easily arrived at and simple to satisfy.

It is a further object of the invention to provide a directional coupler having the foregoing advantages which occupies a relatively small space as compared with prior directional couplers, and has the advantage of universal applicability.

To provide the foregoing and other advantages, the invention contemplates the provision, in a common wide wall between two waveguides of rectangular cross-section, for examplaof individual energy-transferring slots having vmutually 1 perpendicular directions. Energy introduced into one end of one of the waveguides proceeds in part to the other end of that waveguide and in part into and to the correspondingV end of the other waveguide, without change of direction, substantially no energy emerging from the remaining end of the other waveguide.

Other and further objectsv ofthe invention will become apparent from the description of certain embodiments thereof that follows, reference being made to the accompanying drawings, where- Fig. l shows an isometric view partly broken away of a directional coupler in accordance with Fig. 2 shows a top plan view of another embodiment of the invention;

e"mbo ,dinrient ofthe invention;`

- complished by soldering, forl example.

Fig. 4 shows a side view of a waveguide arrangement employing a, directional coupler, illustrating a manner of employment of the present invention;

Fig. 5a is a fragmentary View of a particular embodiment of the invention;

Fig. 5b is a group of graphs illustrating the broad band coupling characteristics of directional couplers like that shown in Fig. 1 having the parameters as shown in Fig. 5a;

Fig. 6 is a graph illustrating the manner in which the coupling characteristic of the invention can be altered with constructions like those shown in Figs. 2 and 3; l l

Fig. 7 is a composite group of graphs illustrating the overall performance -characteristicsof a particular directional coupler, made of elements like those shown in Fig. 1, over a wide band of operating frequencies;

Fig. 8 is a graph illustrating certain characteristics of slot arrangements according to Fig. 2 which are employed to provide uniform coupling over wide operating frequency bands; g

Fig. 9a is a fragmentary View of another particular embodiment of the invention;

Fig. 9b is a group of graphs illustrating the broad band coupling characteristics of certain directional couplers in accordancewith Fig. 2 and having the parameters as shown in Fig. 9a;

Fig. 10 is another composite group of graphs similar to Fig. 7 for a coupler designed in accordance with Figs. 2 and 8;

Fig. 11 shows a top plan View of still another embodiment of the invention; and

Fig. .12 is a group of graphs illustrating the high directivity characteristic of directional couplers ofthe present invention.

Referring now to Fig. 1, a first waveguide l0 and a, second waveguide Il are joined together to provide a four terminal waveguide network. The waveguides may be constructed, as shown in Fig. l, of a pair o-f rectangular U-channels, each of which provides one wide wall and both narrow walls of one waveguide, disposed with the free ends of their respective legs confronting and a plate l2 providing the common wide wall positioned between them. Electrically and mechanically satisfactory joints between the ends of the legs and the long edges of the plate i2 are ac- The precise manner in which the two waveguides are constructed is not material to the invention, however. The parts shown in Fig. 1 may be bolted or riveted together, or, if desired, two conventional waveguides maybe electrically` joined together at their wide walls. Aswill appear below, it is desirable that the common wide wall I2 shall have a minimum thickness, and, for this and other reasons, such as ease of construction of the invention, and smooth nish of the completed article, the construction shown in Fig. l is preferred.

The two waveguides I and II are operated in the lowest or TEM mode, which, as is well known, is characterized by current flow in the wide walls which is largely in the direction of the longitudinal axis of the waveguide along and near the longitudinal center line, and largely in a direction perpendicular thereto in the region of the narrow or side walls. f

The common wide wall I2 is provided with a transversely -directed slot I3 inV theregion of the longitudinal center line and preferably centered thereabout as shown, and a longitudinally directed slot I4 near a side edge thereof. In the embodiment of the inventionshown in Fig. 1, the center points ofthe two slots lie along a common transversely directed liner (not shown), but this is not essential to the invention, as will appear in the discussion of Fig. 3. When one of the waveguides, which will be called the main waveguide, is energized in the 'IE0,1 mode, the two slots being advantageously positioned to interrupt current ow in the wide wall I2, substantial electromagnetic iields are built up about them and energy is thereby coupled into the other waveguide, whichwill `be called the auxiliary waveguide. The slots may be of the same dimensions or, as will be discussed in detail below, theymay `bedifferent. They should preferably not beof lsuch size, however, as to become resonant to the wavesbeing carried, ior the coupling characteristic of the coupler (which will be defined below) changes rapidly with frequency when the slots are resonant to these waves.

vDenominating the open ends of the iirst waveguide lil as terminals I and 3, respectively, and those of the second waveguide I! as terminals .2 and Il, respectively, with terminals I and 2 at one end and terminals` 3 and iat the other end of the network, the networkof Eig. 1 functions as a directionalvcoupler if power incident on terminal I divides in some ratio between terminals 3 and 4 without reaching terminal 2, while power incident on terminal 3 divides between terminals l and 2 without reaching'terminal l, assuming matched output terminals. When'the power incident, for example, at terminal I, is divided evenly between terminals 3 ands, the directional coupler yis called a bridge circuit.

`Denorninating power introduced at any terminal as P1, and power out at terminals I, 2, 3 and 4 as P1, P2, P3, and P4, respectively, the performance of the directional coupler may be speciiied in terms of the coupling, the directivity, and the input voltage standing wave ratio. Where Pi is at terminal I, these terms are defined as'follows:

P4 Couples-eg.

The input voltage standing wave ratio (VSWR) is defined, in the usual manner, as

Vmax Vmin at the input terminal, where V designates voltage. Thefnetwork of Fig. v1 with the single pair `of 4 separate and mutually perpendicular slots I3 and I4 may be considered to be a basic form of directional coupler constructed in accordance with the invention when the output terminals are properly matched. Fig. 2 illustrates how these pairs of slots may be cascaded vto provide a more complex directional co'iiplerl'` 'A' plate l2, which is to fbeemployed, like7 the plate I2 in Fig. 1, as the common wall between two waveguides, is provided with a plurality of transversely directed slots 23 spaced apart along the center line of the plate, and a plurality of longitudinally ,directed slots 24, arranged in two opposing endwise rowsnear the long edges of the plate.

Individual' transverse slots of one alternate series are alignedwith the centers of respective in'dividual'slots'of one row Iof longitudinal slots, while'th'e remaining transverse slots, of a second alternate series, are individually aligned with the Centers 9f lldyidealiraiisverse Slots of the other TOW, Providing, in effect tweseries. Qf alternately disposed pairs 0f Slots. iniersiieitells. arranged- As will be' shown below, increasing the number of pairs of slots increases the coupling ratio, and, to a certain extent, the diretivity. Itwill also be shown that, as long ias the slot pairs are intrinsically reasonably directive, the spacing between sets of Vpairs'is notcritical as far as the coupling ratio is concerned Hence y,the spacing between slot pairscan easily be cho-sen t-orninimize the length of the` Acoupler, and' the .interdigital arrangement 'ofFig 2 is ,valuable to this end. In the embodiment of Eig. 2, the transverse slots Y23 are narrower Athan the longitudinal slots, illustrating a vpractical .embodiment Vt1`iat".has worked well, as Ywill bediscussdinfdetail'lbelow. Fig. 3 illustrates an embodiment of` the invention similar to that of Fig. 'U2 except that lthe transverse and longitudinal slots do not have their centers aligned to form particularly defined pairs of slots." The transverse slots '33 and the longitudinal slots 34 are'` provided in 'a plate v32 similar to the plate I2'jof Fig; l, the transverse slots being varranged along the center lineof the plate, and the longitudinaljslotsbeing arranged in two endwise rows alongboth long edges' of theplate. Fig. 4 shows a manner of employment of directionalv couplers or theinvention, wherein'the main erms provided at its' terminals (and 3 with waveguide coupling'jointsfZu arf1d26`,l re- Spectvgly; `which maybe. 0f. any known'tyne. for connection at terminal I tofja sourcerof power (not shown) and at terminal tofa utilization circuit (likewise not shown). The auxiliary arm 2 I is provided interio'rly at terminal Y`2 'with a matched" termination, inakcc'ordance with known practices, andV at terminal 4"'with another coupling joint 2'I, for connection to a second'utiliz'ation circuit (not showin." By suitable design, power introduced at terminal I'fcan be divided in a large variety of desired ratios v-lrletweeii'the utilization circuitsl connected to'termin'als :3' and 4. due to thefaci, that'tliecouplinsfratio Pff/Pa can 'be altered between wide 'limits' by varying the number oislot pairsandrjth'eir size; The mathematicallcd iderationjs'involved in axis of the waveguide'underJconsi-derationthe 5 field distribution for the TEo,1 mode propagating inthe positive Z-'direction is:

go-sz) e Sill (Relation 1) 2b Y Wei-i2) b e v H Sin Relation 2) 21r WY e? (wt-Taz) (Relation 3) b=the wide cross-sectional dimension of the The `power flow in the positive Z-direction along the wave guide is:

Y (Relation 4) where the integrationris taken over the cross section of waveguide, and

is the complex conjugate of I-IY.

The theory of the coupling of waveguides by Vmeans of small holes in an iniinitesimally thin common `wall between them is developed in reports by I-I. Bethe, entitled Lumped Constants for Small Irises, Radiation Laboratory Report Llil-22, March 1943, and Theory of Side Windows VIAin Waveguides, Radiation Laboratory Report 43-27, April 1943, at page 19 fand following. From these` discussions of Bethe, it follows that for asingle hole the amplitude factor, A, of the lowest mode wave travelling, for example, to the right in the second or auxiliary waveguidev II in Fig. l, when excited by a wave of unit amplitude factor travelling in the same direction in the first or main waveguide Ill, is

#fig (maar MlHeHe- MiHrHr) f (Relation 5) iwhile the v amplitude factor,V B, 'of #the similar plitude electric field in the second waveguide slot.

mode wavetravellingto the left in the auxiliary waveguide H is given by B=2i71(PErEaUfMrHa-M2HeHro1 (Relation e) where:

Eg.) Exil) is the magnitude of the X componentof a unit amplitude electric field as defined in Relation 1 in the' first waveguide Ill evaluated at the center of the hole, and

ESQ

Ex@ is the same term evaluated for unit am- II. y

Hy", HS?, Hz and H? have similar definitions.

P, M1, and Mz are positive real numbers, called polarizabilities, yand are determined by the shape of the hole. They are independent of the wavelength for small holes of which no dimension is greater than M4. M1 and Ma are more particularly called the principal magnetic polarizabilities of the hole, whose axes of symmetry are supposed to lie parallel to the X- and Z- direction, while P is the electric polarizability" of the hole. Sa as used by Bethe is 2S as defined above, and

common wallv I2, of nite thickness is not considered in the reports by Bethe, but may be treated by including in each term of each of Relations 5 and 6 a factor which expresses the voltage attenuation experienced by a mode of the given type travelling 4below cutoff, a distance equivalent to the thickness of the wall.v

As may be seen, consideration of physical arguments indicate that for a narrow slot in a waveguide` wall the only exciting eld which need be considered is one which gives rise to a current tending to flow across the long dimension of the With this fact in mind, it will be seen that each of the slots I3 and I4 in Fig. l couples the two waveguides I0 and II in only one way. The amplitudes At and Bn of the waves excited by the centrally disposed transverse slot I3 are then proportional respectively, to'the second terms on the right-hand side vof each of Relations 5 and 6, or proportional to M1 and M1, respectively. The first and last terms on the right-'hand side of each of these relations may Ibe neglected in the consideration of the transverse slot. Similarly the amplitudes A1.

, and BL of the waves excited by the longitudinally disposed slot I4 are both proportional to may be neglected. The pair of slots I3 and I4 has innite directivity .when the condition is fuliilled, that is, when there is no energy coupled into the auxiliary waveguide II from the main waveguide I0 in the direction reverse to aeeaeoe.

thatfof. thefinan-tuenergyatoethe-vmami.waweguide.

For slots of equal size, MizMz, and consequently )q1-:212, which is the wavelength of thecutoi frequency for the TEM mode. In 1" x 1/2" waveguide with 0.050 wall thickness thisprediets that `infinite directivity occurs, theoretically, ata"wavelength'of"3.25 centimeters.' How this condition Vis urrIodif-ied', inthe 4event tliat'the slots are of different-sizes; orare placed in the waveguide so that the transverse slot I3 does not lie along the center line thereof and/r. the longitudinal slot I4v does not lie adjacent' to vthe side wall ofthe waveguide, is deducible from the foregoing discussion of general principles, and willbe discussedgbelowin connection with certain of the graphicaldata infthedraw.-

( Relation 7) ings..

Thei power flowv from the.mainwaveguide I0 into itheauxiliary waveguide II through a..pair ofslots, I3 andlll, ,forexample as .a .function .of

`frequency is calculated as follows.

Recalling.,that,from Relation .5, it was deduced that, for atransversaslot,

A "ik and substituting'the value of HY asV defined.` in Relation 2 as a definition of unit magnetic leld in both waveguides, it follows that:

(Relation 8) above, that; forrthelongitudinal slot:

a with the substitution fr'ornY Relation 319i' the value of Hz a-s--the unit magnetic'fleld i'n both AL rug-Hang waveguides,l it Vfollows that:

AL-I abj (2b)g.lu2. (Relation 9).

A1. is here the amplitude factorofthe wave coupledby the longitudinalslot I4 and alsov travelling. to the right in the auxiliary waveguide I1.

The total amplitude factor is At-l-Apsince the'4 mutual coupling, of the two slots I3 and I4 is zero. The eld in the. auxiliary. waveguide .is then Ai-I-AL. times the ,respective expressions given in Relations 2 and.3. The ratio of power out of' they auxiliary waveguide (Pi) to thatv entering the main waveguide (Pi) thenxas follows:

M ,y 2 pui-E565. 2 g) (Relation l0) If Mi and M2 are independent of frequency, this,

ratio has a stationary value, and therefore the coupling is stationary, when which-fisRelation '7, the.acondition-.for.maximum directivity. Thus the condition'. for maximum directivity is also the condition for a stationary value of coupling. Also, it Will .be seen that one way to obtain directional couplers which maintain reasonably constant coupling over a wide frequency band is to use small slots, for M1 and M2 are independent of frequency for small slots. Referring now to Figs. .5a and 5b, `the graphs there shown illustrate the manner in which the coupling varies with frequency over the band from 3.1 to 3.5 centimeters, for a single pair of slots 43 and 44, cut lin the commonwall 42 of a directional coupler constructed in accordance with Fig. 1, of 1" x 1/2 waveguides. Theslots are each 0.062" wide, and have the same lengths. Curve A shows that, with slotlengthsiof 0.1875", the coupling was very nearly constant over the band, at avalueof approximately -4'7 db. This means that. with power in at terminal I,

vCurve B' illustrates that, with slot lengths increased to 0.250, the coupling/increased to ape proximately 39' db, but was slightly less stationary in value over the frequency band. Curve C illustrates-that .the coupling can be still further increased in value by increasing the slot length to 0.325, with a little greater sacri'c'e in constancy over. the frequency band, and curve D' shows` that, when the slot .lengths were increased'to 0.450", the coupling increased yto approximately -20 db,fbut suffered a, variation of .approximately 3 db over vthe frequency band. The separation between thetwo slot'sdid not appear to be critical vin value asfar as the coupllng was concerned. Thus the'theoreticalconclusion that the employment of small' slots provides constant coupling over a, wide frequency band is borne out by experiment, but the amount of power coupled by a pair of such slots is low.

Referring; now to Fig. 6, thel single#- curve E there shown illustratesthe mannerin-which the coupling varies when slot pairs like those of Fig. l are cascaded,in a manner shown, for example, in Fig. 2. Again, with power in'. at terminal I, the coupling is defined as The slot pairs investigated to provide curve E constituted each a coupler wherein the coupling andthe curve-illustrates that, when fteenfsucn slotpairs were cascaded, the coupling increased to +23db (approximately).

In' general, it mayl be lstatedl that, when .the coupling of each lslot pair is weak, as` here,

ldoubling the number -of directional' couplers, or

slot pairs, increases the coupling by 6 db, approximately. The-coupling varies essentially as the cotangent ofthe number of' slot pairs. In practice, curve '-E'hasY perrr'iitte'd` prediction, within a1 few tenths of a decibel, offthevcoupling to be expected; of a cascadel of k-nownslot pairs- The'curves shown in Fig. 7 illustrate the-over all performance off a directional coupler having twenty slot pairs, made of identical 15%" x 11g slots, with` 0.2211" separation betweeny the centers of, transverse slots of adjacent pairs. The coupler was investigated over the band 3.1 to 3.5 centimeters, and it. was'foundfrom eurveFv that slot.

lthe coupling varied by less lthan 0.5 db, and was centered about -22 db. Curve G illustrates the directivity characteristic, which it will be noted is about 40 db. The input voltage standing wave ratio (VSWR), shown by curve H, generally diminished as the directivity increased, as was to be expected. Curve G will be discussed in detail below in connection with Fig. 11.

Recalling that a directional coupler constitutes a bridge circuit when Pa-l-.O db it is seen from curve E that approximately 7.7 directional couplers of the -20 db variety must be cascaded in order to obtain a bridge circuit. In many applications, such an arrangement is impractical because the length required is excessive, although the resultant bridge is electrically quite satisfactory.

Generally, the amount of energy coupled by a slot increases as the size of the slot is increased. In particular, from Fig. b, it has been shown that the value of the coupling 1 4 Pa increases toward unity as the slot lengths are increased, but that the relative constancy of this ratio over a. wide frequency band is simultaneously diminished. As the length of a slot approaches its polarizability M increases rather rapidly. 'I'he problem of designing broad band bridge circuits with longer slots and thereby providing acceptably short lengths is thus the problem of choosing a longitudinal slot having a, length such that the change in Mp., (2b)2 with frequency is compensated for by the change in 1\ l 1 M as determined by the length of the transverse From Relation it will be seen that a condition for such compensation is that the slope of the tangent to the curve l@ M shall be the negative of the slope of the tangent tothe curve dx, x, for the transverse slots shall be equal in magnitude and opposite in sense to That is 'J may be obtained by plotting the values o'f At and A1. against kg in a given frequency range in Relations 8 and 9, for each value of M along the ordinate in Fig'. 8, and then plotting the slopes of the resultant curves for each value of M against the value of M. The values of M1 and M2 that are being dealt with are also known from Relations 8 and 9, for At and A1. are measurable voltage or powers, related to a unit input voltage or power, kg is'readily measurable, and the remaining quantities are known constants. Thus for each slot size investigated, the values of M can be determined'initially by measurement and calculation. It will be seen fromthe curves that for small values of M1 (transverse slots) it is possible to choose for each M1 a value of M2 (longitudinal slots) such that the values of the slopes K are equal and opposite, but that, for values of M2 greater than approximately 0.22, this is no longer possible'. For each smaller value o f M2, however, there is a value of M1 which provides frequency compensation to the coupling, and it will be noticed that in all cases the polarizability M2 of the longitudinal slots will have a larger value than the polarizability M1 of the transverse slots.A The polarizability of either` slot can be increased by increasing the length of the slot, as shown in Fig. 5.

Referring now to Figs. 9a and 9b,'the curves L, M, N and O shown in Fig. 9b illustrate the variationxof coupling with frequency for l, 2, 4 and 6 pair of slots, respectively, ofthe sizes shown in Fig. 9a. The slots are cut in a wall 52, of 1" x 1/2, waveguide. Each longitudinal slot 54 is 0.450" long and 0.116" wide. Each transverse slot 53 is 0.436" long and 0.081 wide. The curves L, M, N and O show what is already known from Fig. 6, namely, that the coupling is increased when the number of pairs of slots is increased, but it should be noted in Fig. 9b that each pairV of slots provides a -17 db (approximately) coupler, and only 6 pair are required to provide a bridge, while in Fig. 6, 7.7 of the -20 db pair are required to accomplish the same result. The curves in Fig. 9b illustrate also the fair amount of constancy of coupling versus frequency change that may be attained with the slot arrangement shown.

Fig. 10v illustrates the performance of a directional coupler designed in accordance with the foregoing discussion with relation to Fig. 8. Twenty-four pairs of slots were employed, the center transverse slots being 0.294 long and 0.0707" wide, and the longitudinal side slots being 0.303" long and 0.110" wide. Curve P shows that the coupling was for all practical purposes perfectly constant from 3.2 to 3.5 centimeters, being centered at 0.2 db, which is nearly the condition for bridge circuit. The directivity, curve Q, was flattened out at about 41 db, between 3.2 and 3.3 centimeters, and the VSWR, curve R, was at all times below 1.06. As in all the foregoing examples, all measurements were made on 1 x Ik waveguide. Comparison of Fig. 10 with Fig. ,'7 substantiates the predictions of improved performance attendant upon the discussion in connection with Fig. 8.

Although the theory developed and verified experimentally above is predicated on a single pair of slots which are aligned as in Fig. 1, when a plurality of slots are employed, as in Figs. 2 and 3, there is an averaging of the effects of individual slots, and as a result the alignment of slot'pairs becomes increasingly unimportant as'the number of slots is increased. Fig. 3 illustrates voneembodin'lent ofithe invention-.wherein the Yslots arenot aligned in pairs:. and Fig.` 1l illustrates another. vIn Fig.11,"the widev wall plate 62 shown has aaplurality of .transverse slots 63 which are spaced very closely together, ,the separating metal between thenfr being f of the order of half as wide asvtheslotslthemselves. There are also a plurality of longitudinal slots 64, arranged in rows along the longfsideof 'the .plate 62. Thelongitudinaland transverse-slots arenot related .in pairs. :The staggered relation ofV adjacent rows of' longitudinalslots doesn'ot appear to have electrical significance; it was employed for ease in` cutting the slots. With the embodiment ofy Fig. 11,:the length ofthe coupler for a desired-:coupling ratiov was greatly reduced. The sizes of the individual longitudinal and transverse slots werechosen. in: accordance with the principlesset forth: above in connection Awith Fig. 8, and the results Weresubstantialbf 'pairs of slots,:but' .that there^was noV substantial change of ldirectivitywithin: the; major.- portion .of thefrequency rangeinvestigatedfor and 20 pairs, respectively,` as shown 'byfcurves' U"` and W. However, there was another marked increase in :directivity when the separation between slot pairs was reduced from 0.442" to 0.221 asin 'Fig.'7, and shown bycurveG. On the otherl hand,` it was found that, as long as-.the slotpairs each possessed reasonable directivity in itself, the spacing betweengpairs wasnot criticalfwithrespect to coupling. '-Eorexample,pthe couplings.

of the two twenty-V-pair directional couplers that provided curves 1 W- and .Y Gr'fweren` identical. It should be remembered,;ifrom thediscnssionsfof Figs. 7 and 10,v that when' high directivity; for example db,V or higher: isY obtained,it results yin a correspondingly lowerfinputvoltage standing wave ratio.

In summarizing'it mayi be pointed out that, as far as directivity-andinput VSWR; are .con-

cerned, the directional couplers ofpthepresent y invention are very .uncrtical No `more' than ordinary care' isy requiredxin the; machining or assembly of theparts. 0n the basisfpof experience, as set forth inrdetailhereimthe: problem of constructing adirectional coupler with30-db of directivty overfa 12% band;y and rhaving vany desired coupling, is'a airly-simplematter. In fact, the present Vinvention can providea directional .coupler vhaving v20l db of.. directivi-ty. over frequency .bands approximating theuseful` range of the waveguide itself.

Since numerous variations and `applications within the scope'ofthe invention-willoccur to those skilled' inthe art, itzis intended that the appendedclaims shall bek givena broad interpretation commensurate with their scopeV within the art. The term Vcommonwall? in the claims that follow shall be taken to mean eithera single wall shared by two waveguides or two closely adjacent walls of twoseparate waveguides through 12 which they arercoupled in thek manner taught herein. It willbe remembered in this connection thatthe passage .through al slot lis in a sense a waveguide operated beyond cut-olf and that these passages vshould be maintained as short as possible to avoid excessive attenuation of energy passing through the slot. However, it is realized that the invention could be practiced by providing slot-like passagesl of other than the shortest possible length between two adjacent walls of two separate waveguides.

Having now described my invention, I claim: 1. A waveguide directional coupler comprising a pair of waveguides adapted to convey electromagnetic wave energy and having a common wall, a plurality of transversely directed slots in said common wall spaced apart along the longitudinal vcenter line thereof, and a plurality of separate longitudinally directed slots in said common wall spaced apart along a side edge of said wall, said transversely directed slots being parallel to cach other and substantially perpendicular to said Vlongitudinally directed slots and being disposed in said wall so that a line drawn across said common wall which line passes through a transversely directed slot and is parallel to the long dimension of `said slot also crosses a longitudinally directed slot, the slots in eachV plurality being spaced substantially closer together than one-quarter Aof the wavelength in said waveguides of electromagnetic wave energy intended to be conveyed.

2. A waveguide directional coupler comprising a pair of rectangular waveguides adapted to oonvey electromagnetic wave energy and having a `common wide wall, a transversely directed slot in said wide wall located substantially uniformly about the longitudinal center line thereof, and a separate longitudinally directed slot in said wide wall located near a narrow wall, said slots having portions lying substantially in the same plane transverse to the direction of energy propagation in said waveguides, the size of the latter slot being greater than .that of the former slot, and the relative slot lengths being so proportioned with respect to wavelengths in the operating frequency range that for a particular change in operating frequency the changes in the quantities of energy coupled by the individual slots are. equal in magnitude and opposite in sense, whereby the coupling is maintained substantially constant over the operating frequency range.

3. A waveguide directional coupler comprising a pair of similar rectangular waveguides having a common wide wall, a transversely directed slot in said wide wall located substantially uniform about the longitudinal center line thereof, and a separate longitudinally directed slot in said wide wall located near' a rnarrow wall, the sizes of said slots being so chosen that their principal magnetic polarizabilities, M1 and M2, respectively, are related by the relation 12in/gl YA,- ab MMM,

and M2 isdened by a third relation a'esasoe eax/ t AL ab ,i (21)2M2 where At is the fraction of a unit of input voltage to one of said waveguides which is coupled into the other of said waveguides by said transversely directed slot, AL is the fraction of said unit of input voltage to said one waveguide which is coupled into said other waveguide by said longitudinally directed slot, 7'=\/-1, `pt=perrneability of the transmission medium, e=dielectric constant of said transmission medium, and a is the narrow cross-sectional dimension of said waveguides.

4. A waveguide directional coupler comprising a pair of similar rectangular waveguides having a common wide wall, a transversely directed slot in said wide wall located substantially uniformly about the longitudinal center line thereof, and a separate longitudinally directed slot in said wide wall located near a narrow wall, the sizes of said slots being so chosen that their principal magnetic polarizabilities, M1 and M2, respectively, are so related that is substantially equal in magnitude and opposite in sense to separate longitudinally directed slots in said widewall arranged in rows in the long side areas thereof, said transversely directed slots being parallel to each other and substantially perpendicn ular to sai-d longitudinally directed slots and being disposed in said wall so that'a line drawn across said common wall which passes through a transversely directed slot parallel to the long dimension thereof also crosses a longitudinally directed slot.

6; A waveguide directional couplercomprising a pair of parallel rectangular waveguides adapted to convey electromagnetic wave energy and having a common wide wall, a plurality of parallel transversely directed slots in said wide wall arrayed substantially along the center line thereof, and a plurality of separate longitudinally directed slots in said wide wall each having its center along the axial line of a different )one of said transversely directed slots, said longitudinally directed slots being arranged alternately adjacent to one or the other of the side edges of said wide wall.

7. A waveguide directional coupler comprising a pair of rectangular waveguides adapted to convey electromagnetic wave energy and operative above the cut-off frequency for the 'IEo,1 mode and having a common wide wall, a trans-- versely directed slot of length less than one half a wavelength at said cut-oil frequency in said wide Wall centered substantially at the transverse voltage maximum for said mode, and a longi- 14 tudinally directed slot in said wide wall disposed near a long edge thereof, said slots having portions lying substantially in the same plane transverse to the direction of energy propagation in said waveguides, the latter slot being longer than the former but less than said one half Wavelength long, the relative slot lengths being adjusted to provide that with increasing operating exist in the part of said common wall where said slot is located during passage through either waveguide of electromagnetic waves of a prescribed mode, the slots ofsaid rst group being parallel to each other 'and arrayed side by side along a line parallel to the long dimension of said waveguides, and the slots of said second group being arrayed collinearly in rows on either side of said line.

9. A waveguide directional coupler comprising a pair of waveguides adapted to convey electromagnetic wave energy and having a common wall, and a plurality of pairs of separate mutually perpendicularly directed slots in said common wall, the slots of the first group being arranged side-wise parallel to each other and perpendicular` to the direction of propagation of said energy and the slots of the 'second group being arrayed end-wise parallel to each other and parallel to the direction of propagation of said wave energy, each slot being located to be substantially perpendicular to the current component which may exist in the part of said common wall where said slot is located during passage through either waveguide of electromagnetic waves of a prescribed mode, each slot being short with respect to the wavelength of saidwaves at the operating frequency and non-resonant thereto, and coupling a certain small fraction of the total available power, the number of pairs of mutually perpendicular slots in the coupler being chosen to provide a desired degree of coupling in accordance with the rule that doubling the number of said pairs increases the coupling by approximately six decibels.

i0` A waveguide directional coupler comprising a pair of waveguides having a common wall, and first and second groups of separate mutually perpendicularly directed slots in said common wall. each slot being located to be substantially perpendicular to the current component which may exist in the part of said common wall where said slot is located during passage through either waveguide of electromagnetic waves of a prescribed mode, the slots of said rst group being parallel to each other and arrayed side by side along a line parallel to the long dimension of said waveguides, and the slots of said second group being arrayed collinearly in rows on either side of said line, the space between the slots of said iirst group being of the order of one-half the width of said slots.

11. A waveguide directional coupler comprising a pair of parallel directed similar waveguides having a common wide wall, and a plurality of pairs of` separate Inutzually nerpendicularly ,di rested slots iii-said common wall, said pairs-being arrayed parallel -to the longitudinal direction of said'waveguides, veach slot being located to be substantially perpendicular to the current coinponent which may exist in the part of said common wall where said slot is located during passage through either waveguide of electromagnetic waves of a prescribed mode, each slot being short with respect to the wavelength of said waves at the operating -frequency and non-resonant thereto, and coupling a certain small fraction of the total available-powerpthe number of pairs of mutually perpendicular slots in the coupler being chosen to provide a desired degree ofv coupling in .accordance with the rule that doubling the number-of-said pairs increases the coupling by approximately six decibels.

Q12. A waveguide directional coupler comprising a pairof similar rectangular waveguides operative in the TEM mode and having a common wide wall, a transversely directed slot in said wide wallrcentered substantially at the transverse voltage maximum for said mode, and Va longitudinally 4directed slot in said wide wall near the transverse voltage minimum for said mode, the sizes Aof said slots being so chosen that their principal ,Y magnetic polarizabilities M1 and M2, respectively, are related by the relation ab M2 where At is the fraction of a unit of input voltage to-one of said waveguides which is coupled into the other of said waveguides by said transversely directed slot, AL is the fraction of said unit of input voltage to said one waveguide which is coupled into said other waveguide by said 4 longitudinally,y directed slot, t=pcr- Ineability of the transmission medium, e=dielec tric constant of said transmission medium, and a is the narrow cross-sectional dimension of said waveguides.

di, i,

is substantially equal in magnitude and opposite in sense to i M2M) di, (2W

within a predetermined operating frequency band, where ig is the wavelength of electromagnetic waves ofsaid mode in the waveguides at a particular operating frequency, b is the wide cross-sectional dimension of the waveguides, M1 is dened by a second relation 21'2 s i f A ab u MM,

and M2 is defined byathird relation T2-7. En )w AL- ab ,l (2b)2 2 REFERENCES CITED The following-references are of record in the leof this patent:

UNITED STATES PATENTS Number Name Date 2,423,526 Sontheimer July 8, 1947 2,473,274 Bradley June 14, 1949 2,52,281 Mumford July 3l, 1951 2,573,746 Watson et al Nov. 6, 1951 

